Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIGS. 1A-1C, a SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of a conductive material such as titanium for example. The case 12 typically holds the circuitry and battery 14 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 10 is coupled to electrodes 16 via one or more electrode leads (two such leads 18 and 20 are shown), such that the electrodes 16 form an electrode array 22. The electrodes 16 are carried on a flexible body 24, which also houses the individual signal wires 26 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 18, labeled E1-E8, and eight electrodes on lead 20, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The leads 18, 20 couple to the IPG 10 using lead connectors 28, which are fixed in a non-conductive header material 30, which can comprise an epoxy for example.
As shown in the cross-section of FIG. 1C, the IPG 100 typically includes an electronic substrate assembly including a printed circuit board (PCB) 32, along with various electronic components 34 mounted to the PCB 32, some of which are discussed subsequently. Two coils (more generally, antennas) are generally present in the IPG 100: a telemetry coil 36 used to transmit/receive data to/from an external controller (not shown); and a charging coil 38 for charging or recharging the IPG's battery 14 using an external charger 50. In this example, the telemetry coil 36 and charging coil 38 are within the case 12, as disclosed in U.S. Patent Publication 2011/0112610. (FIG. 1B shows the IPG 10 with the case 12 removed to ease the viewing of the two coils 36 and 38). However, the telemetry coil 36 may also be mounted within the header 30 of the IPG 10 (not shown)).
FIG. 2 shows the IPG 10 in communication with external charger 50 just mentioned. The external charger 50 is used to wirelessly convey power to the IPG 10, which power can be used to recharge the IPG's battery 14. The transfer of power from the external charger 50 is enabled by a coil (antenna) 52. The external charger 50, like the IPG 10, also contains a PCB 54 on which electronic components 56 are placed. Some of these electronic components 56 are discussed subsequently. A user interface, which can include a touchable button 60, an LED indicator 62, a display (not shown) and a speaker (not shown), allows a patient or clinician to operate the external charger 50. A battery 64 provides power for the external charger 50, which battery 64 may itself be rechargeable or replaceable. The external charger 50 can also receive AC power from a wall plug. A hand-holdable case 66 sized to fit a user's hand contains all of the components.
Power transmission from the external charger 50 to the IPG 10 occurs wirelessly and transcutaneously through a patient's tissue 25 via inductive coupling. FIG. 3 shows details of the circuitry used to implement such functionality. For power transmission, control circuitry 70 in the external charger 50 outputs a charging signal (typically, an 80 KHz pulse train) to an amplifier 72 (to “driver circuitry” more generally), which generates a constant AC current Icoil of the same frequency to create an AC magnetic charging field 96. The control circuitry 70 can comprise a microcontroller for example. A capacitor (not shown) is used to tune the resonance of the coil 52 to the frequency of the AC current (e.g., 80 KHz) generated by the amplifier 72. The magnetic field 96 induces a current in the charging coil 38 within the IPG 10, which current is rectified 82 to DC levels, and used to recharge the battery 14, perhaps via a charging and battery protection circuit 84 as shown. When charging the battery 14 in this manner, is it typical that the case 66 of the external charger 50 touches the patient's tissue 25, although this is not strictly necessary.
The IPG 10 can communicate data back to the external charger 50 during charging using reflected impedance modulation, which is sometimes known in the art as Load Shift Keying (LSK). Such back telemetry from the IPG 10 can provide useful data concerning charging to the external charger 50, such as the capacity of the battery 14, or whether charging is complete and the external charger 50 can cease.
Control circuitry 80 in the IPG 10 monitors the battery voltage, Vbat, and with the assistance of LSK modulator 86, produces LSK data. The control circuitry 80 can include a microcontroller for example, and may be associated with Analog-to-Digital (A/D) conversion circuitry to process and interpret the battery voltage. The control circuitry 80 assesses the incoming battery voltage to produce appropriate LSK data at appropriate times. Such LSK data is sent as a serial string of bits to the gate of transistor 88. The LSK data modulates the state of transistor 88, which in turn modulates the impedance of the coil 38. When LSK data=1, the transistor 88 is on (closed) which shorts the coil 38. When LSK data=0, the transistor 88 is off (opened). (Also shown in FIG. 3 are the Frequency Shift Keying (FSK) modulation 92 and demodulation 90 telemetry circuitry coupled to telemetry coil 36, which as noted above, are typically used to communicate with an external controller (not shown)).
Such modulation of the charging coil 38 is detectable at the external charger 50. Due to the mutual inductance between the coils 52 and 38, any change in the impedance of coil 38 affects the voltage needed at coil 52, Vcoil, to drive the prescribed charging current, Icoil: if coil 38 is shorted (LSK data=1), Vcoil increases to maintain Icoil; if not shorted (LSK data=0), Vcoil decreases. In this sense, the impedance modulation of coil 38 is “reflected” back to the charging coil 52, and thus data can be said to be “transmitted” from the IPG 10 to the external charger 50, even if not transmitted in the traditional sense.
Changes in Vcoil are sensed at LSK demodulation circuitry 74 to recover the transmitted LSK data. The serial stream of demodulated bits is then received at control circuitry 70 so that appropriate action can be taken. For example, if the LSK modulation circuitry 86 in the IPG 10 transmits an alternating stream of bits (01010101 . . . ), this might be interpreted by the control circuitry 70 as a stop charging signal, i.e., a signal indicating that the battery 14 in the IPG 10 is full, and therefore that charging can cease. In such an instance, the control circuitry 70 can suspend the production of the magnetic charging field 96 (i.e., setting Icoil to 0), and may notify the user of that fact (by a graphical display, an audible beep, or other indicator).
An issue arising when inductive coupling is used for power transmission relates to the coupling between the coils 52 and 38 in external charger 50 and the IPG 10. Coupling, generally speaking, comprises the extent to which power expended at the transmitting coil 52 in the external charger 50 is received at the coil 38 in the IPG 10. It is generally desired that the coupling between coils 52 and 38 be as high as possible: higher coupling results in faster charging of the IPG battery 14 with the least expenditure of power in the external charger 50. Poor coupling is disfavored, as this will require high power drain (i.e., a high Icoil) in the external charger 50 to adequately charge the IPG battery 14. The use of high power depletes the battery 64 (if any) in the external charger 50, and more importantly can cause the external charger 50 to heat up, and possibly burn or injure the patient.
Coupling depends on many variables, such as the permeability of the materials used in the external charger 50 and the IPG 100, as well materials inherent in the environment. Coupling is also affected by the relative positions of the external charger 50 and IPG 10. The control circuitry 70 uses a coupling detector 76 to detect the alignment or proximity between the external charger 50 and the IPG 10. Typically, the coupling detector 76 includes circuitry that can measure the amplitude of the voltage across the coil 17, which amplitude can be used as an indicator for the degree of proximity and alignment between the external charger 50 and the IPG 10. Coupling detectors 76 are known in the art, and are therefore not discussed in detail here. Additional details concerning alignment detection can be found in commonly owned U.S. Provisional Application No. 61/546,850, filed Oct. 13, 2011 entitled “Charger Alignment in an Implantable Medical Device System Employing Reflected Impedance Modulation.”
Generally, the control circuitry 70 in the external charger 50 indicates misalignment to a user via an alignment indicator 78. Often, the alignment indicator 76 comprises a speaker (not shown) for issuing an audible indication such as a “beep” for example when the external charger 50 is misaligned with the IPG 100. (Alternately, a “beep” could indicate an aligned condition). Alignment indicator 78 can also comprise a visual indicator such as a display or a lamp (e.g., LED 62) on the external charger 50, or a tactile indicator such as a vibration motor that causes the external charger 50 to vibrate. (An audible or tactile indication would be preferred if the external charger 50 isn't easily viewed by the patient during a charging session). Upon hearing, seeing, or feeling (or failing to see, hear, or feel) such an indication, the user of the external charger 50 can use his or her hand to then laterally shift the position of the external charger 50 until better alignment with the IPG 10 is achieved, and the indicator ceases (or issues).
FIG. 4 shows the user interface of the external charger 50. As previously mentioned, the user can turn on/off the charging field 96 by pressing the switch 60. The inventor recognizes drawbacks to having an on/off switch 60 on the charger 50, which drawbacks include increased cost of external charger 50, increased size of user interface, reduced reliability of external charger 50, increased weight of external charger 50, etc.
Solutions have been proposed to allow an external charger to automatically detect when an implant is in its vicinity, and to start charging automatically. For example, in U.S. Patent Application Publication 2009/0112291, an external charger is disclosed that exchanges telemetry with an implant to determine whether charging should begin. In the '291 Publication, the external charger periodically telemeters to the implant requests to begin charging. If the implant is in the vicinity of the external charger, it can receive these requests, and can reply back to the external charger, which can then begin generating a changing field. During charging, the external charger can periodically suspend the charging field to allow the implant to telemeter battery status information, which can allow the external charger to cease producing a changing field once the battery in the implant is fully charged.
The inventor finds the means disclosed in the '291 Publication for automatically determining implant vicinity and automatically beginning charging to be less than optimal, as it requires the external charger to have additional hardware, namely a telemetry transmitter and receiver, and an associated antenna, that are not normally present in an external charger, and which are separate from the external charger's charging coil. Requiring such additional hardware increases the cost and complexity of the external charger.
Additionally, the technique disclosed in the '291 Publication would be ineffective if the battery in the implant has become so depleted that it is unable to operate. If that occurs, the implant would not have enough power to resolve the periodic request signals from the external charger, nor to reply back to the external charger. The external charger would thus conclude that the IPG is not present, and would not provide a charging field, even though the implant clearly needs charging in this circumstance.
It is therefore desired by the inventor that an external charger be able to automatically begin and suspend charging in a solution that does not require substantial modification to the hardware normally present in an external charger, even when the implant battery is depleted, and this disclosure provides such solutions.